The present invention relates to a magnetic field generator for magnetron plasma or, more particularly, relates to a magnetic field generator for magnetron plasma capable of generating a magnetic field having excellent uniformity so that a magnetron constructed therewith is very satisfactory for use in the processes of so-called magnetron sputtering and magnetron etching widely undertaken in the fields of electric and electronic technologies.
It is now conventional that the treatments of sputtering and etching in the above mentioned technologicaI fields are conducted by utilizing the so-called magnetron plasma which is a plasma generated by utilizing a magnetron. Namely, a magnetron plasma is generated by utilizing a magnetron in the following manner. Thus, electrodes are inserted into the atmosphere of a gas such as argon filling a plasma chamber and electric discharge is caused thereby so that the gas filling the chamber is ionized to produce secondary electrons which in turn impinge on the gaseous molecules resulting in further ionization of the gaseous molecules. The primary and secondary electrons generated by the electric discharge enter a drift movement by virtue of the magnetic field generated by the magnetron and the electric field influencing the movement thereof. The electrons under the drift movement can impinge successively on gaseous molecules to cause ionization. thereof along with generation of further electrons which again act to ionize the gaseous molecules by impinging thereon. The very high efficiency of ionization obtained with a magnetron can be explained by the repetition of the above described process.
As is described above, a gas filling a plasma chamber can be ionized with a very high efficiency by utilizing a magnetron consequently increasing the density of plasma generated in the plasma chamber. Accordingly, the efficiency of the magnetron sputtering or magnetron etching can be higher by two to three times than the efficiency obtained in a conventional plasma chamber utilizing high-voltage electric discharge.
FIGS. 1A and 1B schematically illustrate a sputtering apparatus utilizing a conventional magnetron discharge unit by a vertical axial cross sectional view showing the magnetic field and by a perspective view showing movements of the electrons, respectively. Two electrode plates 10, 12 are provided in parallel with each other and a substrate 14, on which sputtering is performed, and a target 16 for sputtering are placed in the space between the electrodes 10, 12, respectively, each in contact with one of the electrodes 10, 12, which are connected to a high frequency power supply. The arrow 20 in FIG. 1A shows the direction of the electric field at a moment when the upper electrode 10 is the anode and the lower electrode 12 is the cathode. A magnetic field generator 18 for magnetron plasma is installed on the lower surface of the electrode 12. The magnetic field generator 18 consists of a yoke 26 and a set of permanent magnets 22 and 24 concentrically arranged and connected together at the lower surfaces by the yoke 26, the outer magnet 22 being annular and the center magnet 24 being cylindrical with the magnetic polarity reversed to that of the outer magnet 22 as is shown in FIG. 1A.
The lines of magnetic force 30 of the fluxes 28A and 28B come out of the N-pole of the outer magnet 22 and enter the S-pole of the center magnet 24 forming a leakage magnetic field above the target 16. Assuming that the direction of the electric field between the electrodes 10, 12 is from the top to the bottom as is shown by the arrow 20, the electron 32 on the surface of the target 16, which should be accelerated straightly upwardly in the absence of a magnetic field, now enters a drift movement by deflection in the direction of the outer product of the electric and magnetic fields along the infinite orbit 34. As a consequence, the electron 32 is under constraint in the vicinity of the surface of the target 16 with promotion of ionization of gaseous molecules. The above is the mechanism which explains the high density of the plasma generated in a magnetron plasma apparatus.
It should be noted here that contribution to the drift movement of electrons can be effected only by the component of the magnetic field in the direction perpendicular to the direction of the electric field. In FIGS. 1A and 1B, namely, only the component of the magnetic field in parallel to the surface of the target 16, referred to as the horizontal magnetic field hereinafter, can contribute to the ionization of the gaseous molecules by bringing the electrons into a drift movement.
FIG. 2A is a graph showing fie distribution of the component of the magnetic field in parallel to the surface of the target, i.e. the horizontal magnetic field, taken in the direction of the arrow X or Y in the magnetic field generator illustrated in FIGS. 1A and 1B. As is understood from the above given explanation for the drift movement of electrons, the magnetic field has an annular distribution so that the distribution curve of the magnetic field in the radial direction has two peaks as is shown in FIG. 2A. Needless to say, the density of plasma generated in the magnetic field depends on the component of, the magnetic field in parallel to the surface of the target, by which the electrons are brought into the drift movement, so that the density of the plasma has a distribution as shown in FIG. 2B by the spot-wise diagram. As a consequence, sputtering with the target proceeds intensely on the areas where the magnetic field has a large component in the direction parallel to the surface of the target to cause localized wearing of the target decreasing the utilization efficiency of the target material which is sometimes very expensive with an economical disadvantage.
The situation is also similar in the apparatus of plasma etching by using a magnetic field generator for magnetron plasma. Namely, the effect of etching proceeds on the surface of a substrate such as a semiconductor silicon single crystal wafer not uniformly but with a localized intensity resulting in a degradation of the quality of the products obtained by the etching treatment. Moreover, it is sometimes the case with such a non-uniform distribution of the plasma density that a phenomenon of so-called charge-up is caused, in which a substantial gradient is produced in the electric potential over the substrate surface, leading to eventual destruction of the semiconductor device under the treatment due to electric discharge.
In view of the above described problems, it is eagerly desired to develop a magnetic field generator for magnetron plasma capable of generating a magnetic field of which the uniformity in the distribution of the component in parallel to the target surface can be increased as high as possible. For example, a magnetic field generator with a dipole ring magnet is known, which, as is illustrated by a plan view and an axial cross sectional view in FIGS. 3A and 3B; respectively, is an assembly of a plurality of anisotropic columnar segment magnets 40, 40 supported within a non-magnetic frame 42 in a circular arrangement.
Different from the magnetic field having an annular distribution of the component in parallel to the target generated in the magnetic field generator illustrated in FIGS. 1A and 1B, the dipole ring magnet of FIG. 3A and FIG. 3B, which shows the cross section as cut and viewed along the direction shown by the arrows IIIB--IIIB in FIG. 3A, generates a magnetic field directed only in one and the same direction in parallel to the surface of the target mounted on the magnet so that the drift movement of electrons proceeds only in one direction not to solve the problem due to the non-uniformity in the distribution of the plasma density. This deficiency of the insufficient uniformity of the magnetic field can be solved partly by rotating the respective columnar segment magnets along the peripheral direction of the circular arrangement thereof although this means alone cannot be effective enough to accomplish full uniformity in the plasma density over a wide range.
The number of the anisotropic columnar segment magnets 40 supported by the non-magnetic frame 42 is at least 8 or, usually, in the range from 8 to 64 although FIG. 3A illustrates the case where the number is 16. The cross sectional profile of the columnar segment magnet 40 is not particularly limitative and can be circular, square, as is shown in FIG. 3A, rectangular or trapezoidal. The arrow mark given in the cross section of each columnar segment magnet indicates the direction of magnetization of the magnet. When the respective columnar segment magnets have the respective directions of magnetization as is indicated in FIG. 3A, a magnetic field in the direction indicated by the broad open arrow mark 43 is generated in the zone surrounded by the set of the columnar segment magnets 40.
When the above described dipole ring magnet is used in a magnetic field generator for magnetron plasma, the upper and lower parallel-plate electrodes 36, 37 are installed in the space surrounded by the columnar segment magnets 40 as is shown in FIG. 3B. The substrate 38 to be subjected to the sputtering treatment and the target 39 are mounted on the lower electrode 37 and upper electrode 36, respectively. The electrodes 36, 37 are connected to a high-frequency power supply so as to generate a high frequency electric field within the space between the electrodes 36, 37. The direction of the thus generated electric field is indicated by the arrow mark 44 or 45 in FIGS. 3A and 3B, respectively, assuming that the upper electrode 36 is the anode and the lower electrode 37 is the cathode at the moment.
It is preferable that the imaginary plane shown by the horizontal broken line 48 in FIG. 3B made by connecting the longitudinal center positions of the columnar segment magnets 40 coincides with the center plane of the effective plasma-working zone 46 having a diameter of 2R and indicated by the broken lines formed on the surface of the target 39 or silicon wafer 38 mounted on the lower electrode 37 by adequately adjusting the height of the target 39, when the treatment is for sputtering, or the wafer 38, when the treatment is for etching: This is because the uniformity of the magnetic field is higher in the longitudinal center height of the columnar segment magnets 40 than in the end portions and the magnetic field in the longitudinal center height of the magnets has in principle no component in the direction perpendicular to the electrode plates so as to accomplish an improved uniformity of the magnetic field and enhanced plasma density.
FIG. 4A includes graphs showing the distribution of the magnetic field generated in the effective plasma-working zone 46 by the magnetic field generator using the above described dipole ring magnet along the lines indicated by the arrows X and Y, respectively, in FIG. 3A, from which it is understood that a substantial improvement can be obtained in the uniformity of the magnetic field in the horizontal direction as compared with the distribution shown in FIG. 2A. On the other hand, FIG. 4B shows a distribution diagram of the density of plasma generated in the magnetic field, which indicates non-uniformity of the plasma density being higher in the negative side of the arrow Y and lower in the positive side of the arrow Y. This phenomenon can be explained by the fact that the drift movement of electrons takes place in the direction of the outer product of the electric and magnetic fields so that the electrons proceeds towards the negative side of the arrow Y in FIG. 4B. This is the reason for the problem that, although the direction of the drift movement of electrons can be changed by the rotation of the dipole ring magnet along the peripheral direction of the circular arrangement of the magnets, full uniformity of the plasma density cannot be accomplished in a wide range by merely rotating the dipole ring magnet.